1. Field of the Invention
This invention relates to a heat-resistant alloy based on an intermetallic compound TiAl, which is suitable for use as a light-weight heat-resistant material. More specifically, it relates to a heat-resistant alloy based on an intermetallic compound TiAl, which has improved mechanical strength, ductility at room temperature and strength at high temperatures.
2. Description of the Prior Art
It is known that an intermetallic compound TiAl (to be referred to as a TiAl phase) in which about 35 to 60% by weight of aluminum has a crystal structure Ll.sub.o exists in a titanium-aluminum binary system. The TiAl phase has excellent properties among which are:
(1) it is light in weight; PA1 (2) it has good oxidation resistance at high temperatures; PA1 (3) its strength increases with increasing temperature, and becomes maximum at about 700.degree. C.; and PA1 (4) it has good creep strength at high temperatures.
However, it has poor ductility at room temperature and is difficult to deform plastically by conventional fabricating machines because of the strong dependence of plasticity on the strain rate at high temperatures. For this reason, the TiAl phase has not gained practical acceptance. Attempts have been made to solve these problems and to have the TiAl phase exhibit its excellent properties by adding various third and fourth elements which can dissolve in the TiAl phase, or by dispersing a second phase in addition to the TiAl phase. Known intermetallic compound TiAl-base alloys successfully having improved ductility at room temperature are a Ti-34.1% by weight Al-3.4% by weight V alloy (U.S. Pat. No. 4,294,615) and a Ti-41.7% by weight Al-10% by weight Ag alloy (Japanese Laid-Open Patent Publication No. 123847/1983). The alloy of the U.S. Patent having improved ductility has an elongation of only about 2% at room temperature, and it is desired to improve its ductility further. Furthermore, its strength at high temperature is not entirely satisfactory. The Ag alloy, on the other hand, has greatly improved ductility at room temperature, but has markedly reduced strength at temperatures exceeding 600.degree. C. Such an alloy is unsuitable as a high-temperature heat-resistant material.